In industry, there often arises a need to precisely measure the spatial location of a point or points on an object. This need may occur, for example, in operation of coordinate measurement machines, machine tools, and robots. Typically, a probe is mounted on the machine for movement in three dimensions, X, Y, and Z. The machine is capable of measuring the X, Y, and Z coordinates of the probe.
Two types of probes are in common use. One is a trigger probe, which produces a trigger signal when a stylus attached to the probe contacts the workpiece surface. This trigger signal is used to latch the output from the scales and read heads of the machine, and causes a computing device associated with the machine to read the X, Y, and Z coordinates thus sensed, which indicate the position of the point where the stylus contacted the workpiece.
The other type of probe is a measurement probe. The measurement probe produces an analog or digital output proportional to displacement of a stylus of the probe in the X, Y, and Z directions, rather than merely producing a trigger signal. Such probes are particularly useful for scanning the contour of a workpiece surface, as opposed to measuring specific points on the surface. It is in the measurement-type probe that the present invention finds its primary application.
Typical prior art measurement probes have a pair of spring members located at either end of a member movable along each of the X, Y, and Z axes. For example, see WIPO Publication WO89/05435 of McMurtry et al., and FIG. 2a. The stylus is mounted to the spring members so that movement of the stylus in the X, Y, and Z directions produces corresponding movement in the respective axial members. This movement is measured by precision position sensors associated with each axis. In some cases, these position sensors have stop elements or biasing elements for use in establishing stable rest positions, as illustrated for example in U.S. Pat. Nos. 4,578,873, 4,716,656, and 5,029,399.
FIG. 1 illustrates, with some exaggeration for illustrative purposes, a phenomena found in many conventional measurement probes. Spring members 10 for each axis form a rectangle 20 which is distorted by movement of the stylus 30 along that axis, so that the rectangle becomes a parallelogram 40. Because the dimensions of the spring members 10 are fixed, as the spring distortion increases due to movement along one axis, this parallelogram effect produces a measurable apparent movement "d" along an axis perpendicular to the axis of spring distortion. Thus, in this type of probe, some of the measured movement in each axis is the result of spring deformation within the probe, rather than any actual variation in position of the workpiece along that axis. The amount of error resulting from spring geometry increases with increased displacement of the springs along a given axis.
The 3D Accuprobe 3.0 measurement probe, designed and manufactured by the assignee, Automated Precision, Inc. of Gaithersburg, Md., provided an improved internal geometry enabling significant improvements in measurement accuracy. This probe incorporates an improved V-spring mechanism (FIG. 2b) that reduces distortion effects of the spring geometry.
However, in situations where very precise position measurements are desired, even with improved spring geometries, the springs bias the stylus into an arc-shaped path. Thus, even if the relative motion of the stylus and workpiece occurs only in a single axis, the spring geometry creates a movement measurement in an axis perpendicular to that axis, generating a measurement error along the perpendicular axis. If it is known that movement along an axis should not occur based on the configuration of the workpiece, it is possible to ignore measured movements along that axis. However, the flexure of the spring members and resulting movements within the probe may contribute to a measurement error component along the axis of interest, in addition to creating false movement readings on other axes.
Thus, there has arisen a need in industry for a precision measurement probe in which movement along the various axes can be selectively prevented, by locking the stylus so that it will not move along one or more selected axes. However, a compact, effective locking measurement probe has not heretofore been available.